1. Field of the Invention
The present invention relates to color image forming apparatuses, such as copying machines and printers of electrophotographic types, electrostatic recording types, and the like, and sensors usable in such color image forming apparatuses. Particularly, the present invention relates to shading correction in those color image forming apparatuses and sensors.
2. Related Background Art
FIG. 8A exemplifies a sensor for detecting light reflected by a toner patch, which uses a photodiode. FIG. 8B exemplifies a circuit for converting an output current of the photodiode into a voltage. The photodiodes 201-R, 201-G, and 201-B, receive light transmitted through red (R), green (G), and blue (B) color filters 202-R, 202-G, 202-B, respectively. Denoted at 105 is an LED serving as a light source. Denoted at 104 is a toner patch of a detection object formed on a transferring material 1. Light components transmitted through the R, G and B color filters 202-R, 202-G, 202-B out of reflected light 203 from the toner patch 104 enter the photodiodes 201, respectively, and photocurrent is generated in each photodiode. The photocurrent is converted into a voltage by each resistor 204-R, 204-G or 204-B, and the voltage is amplified by each amplifier 205-R, 205-G, or 205-B to create an output voltage V206-R, V206-G or V206-B.
FIG. 9 exemplifies another sensor for detecting light reflected by the toner patch 104. The sensor of FIG. 9 is different from the sensor of FIG. 8 in that light diffracted by a diffraction grating 208 without using any color filters is detected by a photodiode array 207 (207-1 to 207-n) comprised of an n number of pixels. Colors (R, G and B light components, or spectral outputs in respective wavelength ranges) of the toner patch 104 formed on the transferring material 1 can be detected by using those sensors or pixels.
On the other hand, in the case of an image forming apparatus of an electrophotographic type using an intermediate transfer member, obtainable density and chromaticity of an image are likely to fluctuate if variations occur in each portion of the apparatus due to changes in its ambience and its longtime use. Particularly, in the case of a color image forming apparatus, there is a fear that colors lose their balance even in the event of slight fluctuations in their densities, and accordingly constant density and gradation need to be maintained for each color at all times.
Therefore, for each color toner, there are prepared several kinds of process conditions of exposure amount, developing bias, and the like corresponding to respective absolute humidities, and a gradation correcting unit such as a lookup table (LUT). Based on the absolute humidity measured by a temperature-humidity sensor, appropriate process condition and gradation correction value are selected on each occasion. Further, in order to obtain constant density, gradation, and color tint even if variations occur in each portion of the apparatus during its use, a toner image (also referred to as a patch or a toner patch) for detecting the density is formed with each toner on the intermediate transfer member, and the patch is detected by a sensor. Thus-detected results are fed back to the process conditions of the exposure amount, the developing bias, and the like to control the density of each color such that a stable image can be obtained.
Furthermore, Canon has proposed a sensor for detecting the color tint of a patch fixed on the transferring material such that feedback can be executed with respect to factors including influences of transfer and fixation which are excluded from feedback objects in the above-discussed density detecting sensor, and influence at the time of mixing colors which cannot be detected. Based on results detected by this sensor, feedback operations are performed to the process conditions and image processing such that color stabilization of the image can be further improved.
However, when the color tint of the patch fixed on the transferring material is detected to obtain a stable image in a color image forming apparatus using the conventional sensor as illustrated in FIGS. 8A, 8B or FIG. 9, the following problems arise.
In both cases where the color tint is detected using plural color filters and plural photodiodes as illustrated in FIG. 8A, and where a spectrum of the light reflected by the patch is created by the diffraction grating or prism, and the spectrum light is detected by plural sensors to detect the color tint of the patch as illustrated in FIG. 9, errors are likely to occur in detected colors due to dispersions or variations in sensitivities of the sensors, resistance values of the IV (current-voltage) converting resistors, transmissivities of the color filters, and amounts of light depending on positions of the patches and the sensors.
In normal sensors, shading correction is carried out to compensate for those dispersions. More specifically, light reflected by a white-color reference board is read to obtain and store a coefficient for making the output from each pixel of the sensor constant for each pixel, and the individual detected results are corrected using these coefficients. However, it is difficult in the image forming apparatus to stably maintain the color tint of the reference board for a long time due to contamination by the toner and a change in color of the reference board with age. In addition, preparation of the white-color reference board itself results in an increase in the cost. Further, although it can be considered that the transferring material is used as the reference, the transferring material is difficult to use as the reference for the color-tint detecting sensor since the color of the transferring material is not always white.
As described in the foregoing, in conventional shading correction methods as discussed above, the cost is likely to rise, and precision of detected information is liable to lower since stable shading correction cannot be achieved. Accordingly, in the image forming apparatus capable of controlling operation based on such information, precision of its color stabilizing control is likely to decrease.